An NOx selective reduction catalyst (SCR) is in some cases used in an exhaust gas purifying apparatus for reducing nitrogen oxides (NOx) emitted from, for example, a diesel powered automobile, and a urea aqueous solution is used as its reductant. It is known that a urea aqueous solution with a urea concentration of 32.5 wt % can be advantageously used for effectively performing this reducing reaction.
However, in the urea aqueous solution accommodated in a urea water tank mounted in a diesel powered automobile, there are cases where the urea concentration changes due to such as a change over time. In addition, there is a possibility of a different type of solution (such as light oil) or water becoming erroneously mixed into the urea water tank. In view of such circumstances, liquid state detecting sensors have been proposed to manage the state of the liquid (such as the urea concentration of the urea aqueous solution) in the urea water tank (refer to JP-A-2005-84026 (corresponding to US2007/0054409A1), JP-A-2005-127262 (corresponding to US2007/0209428A1) and JP-A-2005-337969 (corresponding to EP1752762A1), for example).
The liquid state detecting sensors in JP-A-2005-84026 (corresponding to US2007/0054409A1), JP-A-2005-127262 (corresponding to US2007/0209428A1) and JP-A-2005-337969 (corresponding to EP1752762A1) have an element (state detecting element) in which a substrate, a temperature sensing element, an insulating layer, and a heating element are sequentially stacked. In this liquid state detecting sensor, the heating element is energized for a predetermined time duration, and the urea concentration and the type of the liquid to be measured are detected on the basis of a temperature change of the heating element measured by a temperature sensing element before and after the energization. Specifically, since a difference arises in the heat capacity of the urea aqueous solution due to a difference in the concentration of urea contained in the urea aqueous solution, a difference arises in the temperature change of the heating element due to the difference in the urea concentration. The urea concentration and the type of the liquid to be measured are detected by detecting the temperature change of the heating element by making use of this principle.
In cases where the urea aqueous solution in the urea water tank has decreased, and its liquid level has dropped below the position of the element (heating element), or in cases where a liquid (such as light oil) whose thermal conductivity is smaller than that of the urea aqueous solution is accommodated in the urea water tank, the rate of temperature increase of the heating element becomes high, so that there is a possibility of the heating element undergoing an abnormal temperature rise. In such a situation, if the heating element is energized for a predetermined time duration (4 seconds and 8 seconds are respectively cited by way of example in JP-A-2005-84026 (corresponding to US2007/0054409A1) and JP-A-2005-337969 (corresponding to EP1752762A1)) as in the case where an appropriate amount of the urea aqueous solution is accommodated in the urea water tank, there has been a possibility of the heating element undergoing an excessive temperature rise, resulting in the failure (breakage or the like) of the element.
It should be noted that it is described in JP-A-2005-84026(corresponding to US2007/0054409A1) that in cases such as where the urea aqueous solution in the urea water tank has decreased, and its liquid level has dropped below the position of the element (heating element), an alarm can be issued by the liquid state detecting sensor. However, since the heating element has already undergone the excessive temperature rise for a long time period when the alarm is issued, it has been impossible to prevent the failure of the element ascribable to the excessive temperature rise.